In a wireless communication system, especially in a case in which a wireless communication is performed by using a microwave, it is effective to adopt a SD (Space Diversity) method as a measurement to prevent fading.
SD method is a technology to compensate for degradation of radio wave propagation characteristics due to fading, by using a low possibility in which received waves in a plurality of antennas arranged with appropriated distances be degraded at a same time.
FIG. 1 is a diagram which schematically shows a configuration example of a wireless communication system using SD method. The wireless communication system shown in FIG. 1 includes a first fixed station 2001 and a second fixed station 2002. The first fixed station 2001 includes a first antenna and a second antenna. The second fixed station 2002 includes a third antenna. In the first fixed station 2001, the first antenna and the second antenna are separated with a distance d between their centers from each other. Here, as an example, the first antenna, the second antenna and the third antenna are arranged in a horizontal direction to face each other; the first antenna is disposed at an altitude L at the center from the ground; the second antenna is disposed at an altitude L+d at the center from the ground; and the third antenna is disposed at an altitude L′ at the center from the ground.
In the wireless communication system shown in FIG. 1, the second fixed station 2002 transmits a signal from the third antenna and the first fixed station 2001 receives this signal at the first antenna and the second antenna. The first fixed station 2001 performs a receiving process including a diversity treatment over two signals obtained by the first antenna and the second antenna, respectively.
FIG. 2 is a circuit diagram which shows an example of a configuration of a circuit which performs a receiving process in a wireless communication system using SD method. The circuit in FIG. 2 includes a first antenna, a second antenna, a first mixer, a second mixer, a first A/D (Analog to Digital) converter, a second A/D converter, a diversity combination section and a local oscillator. It should be noted that, in the circuit diagram in FIG. 2, quadrature demodulation π/2 component and filter thereof are omitted. Here, the first antenna and the second antenna in FIG. 2 respectively correspond to the first antenna and the second antenna in FIG. 1. A group of the first antenna, the first mixer and the first A/D converter is called the first branch. Similarly, a group of the second antenna, the second mixer and the second A/D converter is called the second branch.
A first signal received by the first antenna is provided to the combining circuit through the first mixer and the first A/D converter. Similarly, a second signal received by the second antenna is provided to the combining circuit through the second mixer and the second A/D converter. Here, the group of the first antenna, the first mixer and the first A/D converter, which are connected in series, is called the first branch. Similarly, the group of the second antenna, the second mixer and the second A/D converter, which are connected in series, is called the second branch. Finally, the combining circuit combines two signal provided from the first branch and the second branch to terminate the diversity process.
Relating to the above, a patent literature 1 (Japanese Laid Open Patent Publication Heisei 9-331281) discloses a description about a transmission and reception apparatus. This transmission and reception apparatus includes an antenna alternatively used for transmissions and receptions by a plurality of branches, a base band demodulating section and a transmission antenna switching section. Here, the base band demodulating section demodulates a received signal of each branch. The transmission antenna switching section switches transmission antennas based on reception power of received signal of each branch. This transmission and reception apparatus performs transmission diversity in a wireless communication system with TDD method. This transmission and reception apparatus is characterized for including a comparing means. Here, the comparing means compares the reception power of the received signal of each branch by use of digital data obtained in the demodulation process of the base band demodulating section and outputs selection information of transmission antenna to the transmission antenna switching section.
Also, a patent literature 2 (Japanese Laid Open Patent Publication 2004-518331) discloses a description about an adaptive antenna system. This adaptive antenna system is characterized for including N antennas, N forward equalizers and N processors. Here, N forward equalizers are respectively connected to N antennas in an operational manner. N processors execute a constant modulus algorithm to respectively generate N control signals to adjust factors associated to each of N forward equalizers.
Also, a patent literature 3 (Japanese Laid Open Patent Publication 2005-94500) discloses a description about a wraparound canceller for diversity reception. This wraparound canceller for diversity reception has a diversity receiving means for OFDM signal and a wraparound cancelling means. Here, the diversity reception means for OFDM signal has a plurality of branches. The wraparound cancelling means is characterized for inputting a received signal after diversity combination outputted from the diversity reception means and outputting after practically cancelling a wraparound wave.
Also, a patent literature 4 (Japanese Laid Open Patent Publication 2008-48139) discloses a description about a microwave wireless transmission and reception apparatus. This microwave wireless transmission and reception apparatus is used as a first fixed station of a wireless communication system which performs a wireless communication by use of a microwave between the first fixed station in which a plurality of antenna is arranged and a second fixed station in which a single antenna is arranged. This microwave wireless transmission and reception apparatus is characterized for including a reception processing means, a transmission processing means, a selecting switch and a selection controlling means. Here, the reception processing means is arranged in correspondence with each of the plurality of antenna and performs a reception process on a microwave signal arrived to the corresponding antenna. The selecting switch transmits a radio frequency signal, obtained from the transmission processing means, to an antenna specified in accordance with a selection controlling signal which specifies one among the plurality of antennas, to provide to a wireless transmission by the antenna. The selection controlling means synchronizes with a selection among the received signals respectively obtained by a plurality of reception processing means, generates the selection controlling signal which specifies the antenna corresponding to the selected received signal to input to the selection switch.